The present invention relates to an illumination device for illuminating at least one spatial light modulator device being used preferably for displaying two-dimensional and/or three-dimensional images comprising at least one light source device with at least one light source for illuminating the at least one spatial light modulator device, a preferably substantially planar light guiding element comprising a light conducting core, where the at least one light source device is arranged on a side of the light guiding element and where the light emanating from at least one light source of the at least one light source device propagates areal through the light guiding element, and at least one light decoupling element arranged on top or inside of the light guiding element, the at least one light decoupling elements being provided for decoupling of a wave field of the light which propagates in the light guiding element into the direction of the at least one spatial light modulator device. The invention relates further to a method for measuring the intensity distribution of light existing at an entrance plane of a spatial light modulator device.
Particularly, the present invention relates to an illumination device which is used to illuminate at least one spatial light modulator device being preferably applied in a display for displaying two-dimensional (2D) and/or three-dimensional (3D) images. It shall be understood that two-dimensional images and three-dimensional images also include two-dimensional or three-dimensional contents or movies.
The thin illumination device according to the invention can be used, for example, in a stereoscopic display device, an autostereoscopic display device (ASD) or a holographic display device, in particular for a mobile holographic three-dimensional display device or a larger holographic or auto-stereoscopic display device.
Illumination devices can be provided as backlights or frontlights (also referred to as transmitted-light and reflected-light illumination devices, respectively) and generally serve to illuminate a light-transmissive or reflective controllable spatial light modulator device (SLM) of preferably a direct-view display device. The light can be coherent or incoherent. Display devices which are operated with incoherent light are preferably used as two-dimensional displays for autostereoscopic three-dimensional presentations. Coherent light is required, for example, in holographic display devices.
The field of application of the present invention includes preferably direct-view display devices for the three-dimensional presentation of autostereoscopic and/or holographic images.
In a commercially available flat TV display for the presentation of two-dimensional images or movies/videos, it is necessary to realize a bright and homogeneous illumination of the entire surface at high resolution. The spatial light modulator device which serves as display panel is required to emit the light in a large angular range. Many physical forms of such display devices are known in the prior art.
Most of them have a planar optical light guiding element/waveguide. The planar optical light guiding element generally comprises at least one light conducting core and a cladding layer. The injected light propagates through the planar optical light guiding element in the form of light beams or wave fields under the conditions of total internal reflection (TIR) and is coupled out to illuminate the spatial light modulator device. Alternatively, the light is conducted without being reflected and coupled out through the cladding layer as evanescent wave fields of different modes m.
A number of issues need to be considered in a display device with backlight or frontlight and preferably planar optical light guiding element to be able to realize an optimally designed illumination device. First, this relates to the physical form of a preferably planar optical light guiding element itself, including the mechanisms for injecting and coupling out the light. Secondly, this relates to the physical form of the light source device including the light sources which supply the light.
In contrast to a flat TV display, an illumination device in an autostereoscopic or holographic display device for the three-dimensional presentation of information has to satisfy a number or further or different requirements. The information to be presented is written into the spatial light modulator device of the display device. The light which is emitted by the light source is modulated with the information that is written into the spatial light modulator device, where the spatial light modulator device often at the same time serves as screen or display panel. It is therefore necessary to strictly ensure parallel incidence of the light beams onto the spatial light modulator device and to achieve a high refresh rate of the spatial light modulator device.
In addition to the necessary high refresh rate, great demands are made on the collimated emission of the light by the optical light guiding element. To achieve a high quality of the three-dimensional presentation of the information written into the light modulator device, a defined collimation of the wave fronts that are coupled out is necessary in addition to a homogeneous illumination of the entire surface of the spatial light modulator device. This is of particular importance for holographic presentations in the form of a reconstruction that is to be generated. The holographic information, which can for example be an object that is composed of object points of a three-dimensional scene, is encoded in the form of amplitude and phase values in the pixels of the spatial light modulator device. Each encoded object point contributes to a wave front that is emitted by the spatial light modulator device.
The angular range of a wave front that is emitted by the illumination device is referred to as the ‘angular spectrum of plane waves’ (ASPW). It has been found in practice that an angular spectrum of plane waves where the plane wave fronts comprise mutual deviations in the emission angle of more than 1/60° deg in the direction of coherent reconstruction will result in a blurred reconstructed object point. This blur can be perceived by the human eye under optimum conditions. The emission angle of the angular spectrum of plane waves of a holographic display device should therefore lie at least in the range of between 1/70° deg and 1/40° deg in the coherent direction. In the incoherent direction, it should be wide enough to illuminate at least the eye pupil of the human eye.
Consequently, the collimated wave fronts which illuminate the spatial light modulator device have to a priori have a defined emission angle in relation to each other in order to circumvent the negative illumination-induced effects on the reconstruction to be generated. In autostereoscopic three-dimensional presentations, the collimation of the light beams enhances the image quality of the display device. The angular spectrum of plane waves should here be chosen such that the eye pupil of the other human eye is not illuminated if one eye pupil is illuminated.
Collimated emission of coherent light can for example be achieved by using volume gratings which are arranged on or in the preferably planar optical light guiding element. They represent a stack of transparent layers and can be described as modulated distributions of refractive indices in the X and Y direction. A three-dimensional volume grating is generated by interference of two or more coherent or at least partly coherent waves. The structure of the volume grating is determined by parameters such as the wavelength in the material and the local angles between interfering wave fronts of the light used for recording. A volume grating is generally made such that a defined portion of energy can be coupled out in a specified angular range. Bragg's diffraction conditions apply to those gratings during reconstruction.
An adaptation to the light that is actually to be coupled out can be achieved by choosing the parameters of the volume gratings accordingly.
Further, the resolving power limit of the human eye, which is about 1/60° deg, has to be taken into account when producing the volume grating. If this limit is taken into account, the illumination device e.g. in a holographic display device has to realize an angular spectrum of plane waves that ranges between 1/20° deg and 1/60° deg in order to illuminate the spatial light modulator device with well collimated light.
Furthermore, the problem is to realize a flat illumination device which is as thin as possible. This means the illumination device should have a thickness which is suited preferably for a holographic display device. As mentioned briefly above, the angular resolution of the human eye under optimal conditions is 1/60° deg. The illumination device of a holographic display device therefore has to have a limited angular spectrum of plane waves, e.g. from < 1/20° deg to minimally 1/60° deg, that is it must be well collimated light. Therefore, the basic boundary condition is that the flat illumination device according to the invention shall provide 1/60° deg angular spectrum of plane waves (ASPW) only, which is present in the direction of holographic encoding. In detail, for a holographic encoding, a one-dimensional (1D) encoding requires 1/60° deg along the coherent direction and e.g. 1° deg along the incoherent direction. According to this a two-dimensional encoding requires 1/60° deg in horizontal and vertical direction. This wave field propagates then to at least one observer of a two-dimensional and/or three-dimensional image. Moreover, at present reasonable thin time being illumination devices realize an angular spectrum of plane waves of plus/minus 30° deg, which is far away from being practical.
There are well-known different approaches which try to resolve this problem. One prior art solution is a wedge type illumination device. This illumination device comprises a wedge-shaped light guiding element, i.e. one which is not coplanar, in which the light propagates by way of multiple reflections and which is used for homogeneous illumination of a display. Further, the wedge is dimensioned such that the light leaves because of the frustrated total internal reflection (FTIR) condition during its propagation through the light waveguide.
Diffractive wedge type embodiments of the light guiding element in an illumination device can provide reasonable flat illumination devices. The problem with such illumination devices comprising a wedge-shaped light guiding element is that they use a primary collimated wave field, which enters in a plane of a substrate as the light guiding element, which e.g. comprises an antireflection coating. To realize larger entrance angles as e.g. 87.134° deg, which gives approximately a twentyfold (20×) beam stretching, is very difficult and could be the limit for the wedge type approach. In other words, a large incidence angle must be chosen in order to enable a large beam stretching factor. An angle of e.g. 84.26° deg, which means 84.26° deg incidence angle to 0° deg exit angle to the surface normal of the light exit plane of the illumination device and which is present between the normal of the diffraction plane and the incidence beam, generates a beam stretching factor of 1/cos(84.26° deg)=10. On the other hand, an entrance angle of 86.18° deg generates then a fifteenfold (15×) and an entrance angle of 87.13° deg generates a twentyfold (20×) stretching factor.
Therefore, flat displays with light guiding elements designed in the form of a wedge are not suited due to their emission characteristics to satisfy the great demands which are made on an illumination device of a fast large sized switching display device.